1 // SPDX-License-Identifier: GPL-2.0-only
2 /*
3 * sigreturn.c - tests that x86 avoids Intel SYSRET pitfalls
4 * Copyright (c) 2014-2016 Andrew Lutomirski
5 */
6
7 #define _GNU_SOURCE
8
9 #include <stdlib.h>
10 #include <unistd.h>
11 #include <stdio.h>
12 #include <string.h>
13 #include <inttypes.h>
14 #include <sys/signal.h>
15 #include <sys/ucontext.h>
16 #include <sys/syscall.h>
17 #include <err.h>
18 #include <stddef.h>
19 #include <stdbool.h>
20 #include <setjmp.h>
21 #include <sys/user.h>
22 #include <sys/mman.h>
23 #include <assert.h>
24
25
26 asm (
27 ".pushsection \".text\", \"ax\"\n\t"
28 ".balign 4096\n\t"
29 "test_page: .globl test_page\n\t"
30 ".fill 4094,1,0xcc\n\t"
31 "test_syscall_insn:\n\t"
32 "syscall\n\t"
33 ".ifne . - test_page - 4096\n\t"
34 ".error \"test page is not one page long\"\n\t"
35 ".endif\n\t"
36 ".popsection"
37 );
38
39 extern const char test_page[];
40 static void const *current_test_page_addr = test_page;
41
sethandler(int sig,void (* handler)(int,siginfo_t *,void *),int flags)42 static void sethandler(int sig, void (*handler)(int, siginfo_t *, void *),
43 int flags)
44 {
45 struct sigaction sa;
46 memset(&sa, 0, sizeof(sa));
47 sa.sa_sigaction = handler;
48 sa.sa_flags = SA_SIGINFO | flags;
49 sigemptyset(&sa.sa_mask);
50 if (sigaction(sig, &sa, 0))
51 err(1, "sigaction");
52 }
53
clearhandler(int sig)54 static void clearhandler(int sig)
55 {
56 struct sigaction sa;
57 memset(&sa, 0, sizeof(sa));
58 sa.sa_handler = SIG_DFL;
59 sigemptyset(&sa.sa_mask);
60 if (sigaction(sig, &sa, 0))
61 err(1, "sigaction");
62 }
63
64 /* State used by our signal handlers. */
65 static gregset_t initial_regs;
66
67 static volatile unsigned long rip;
68
sigsegv_for_sigreturn_test(int sig,siginfo_t * info,void * ctx_void)69 static void sigsegv_for_sigreturn_test(int sig, siginfo_t *info, void *ctx_void)
70 {
71 ucontext_t *ctx = (ucontext_t*)ctx_void;
72
73 if (rip != ctx->uc_mcontext.gregs[REG_RIP]) {
74 printf("[FAIL]\tRequested RIP=0x%lx but got RIP=0x%lx\n",
75 rip, (unsigned long)ctx->uc_mcontext.gregs[REG_RIP]);
76 fflush(stdout);
77 _exit(1);
78 }
79
80 memcpy(&ctx->uc_mcontext.gregs, &initial_regs, sizeof(gregset_t));
81
82 printf("[OK]\tGot SIGSEGV at RIP=0x%lx\n", rip);
83 }
84
sigusr1(int sig,siginfo_t * info,void * ctx_void)85 static void sigusr1(int sig, siginfo_t *info, void *ctx_void)
86 {
87 ucontext_t *ctx = (ucontext_t*)ctx_void;
88
89 memcpy(&initial_regs, &ctx->uc_mcontext.gregs, sizeof(gregset_t));
90
91 /* Set IP and CX to match so that SYSRET can happen. */
92 ctx->uc_mcontext.gregs[REG_RIP] = rip;
93 ctx->uc_mcontext.gregs[REG_RCX] = rip;
94
95 /* R11 and EFLAGS should already match. */
96 assert(ctx->uc_mcontext.gregs[REG_EFL] ==
97 ctx->uc_mcontext.gregs[REG_R11]);
98
99 sethandler(SIGSEGV, sigsegv_for_sigreturn_test, SA_RESETHAND);
100
101 return;
102 }
103
test_sigreturn_to(unsigned long ip)104 static void test_sigreturn_to(unsigned long ip)
105 {
106 rip = ip;
107 printf("[RUN]\tsigreturn to 0x%lx\n", ip);
108 raise(SIGUSR1);
109 }
110
111 static jmp_buf jmpbuf;
112
sigsegv_for_fallthrough(int sig,siginfo_t * info,void * ctx_void)113 static void sigsegv_for_fallthrough(int sig, siginfo_t *info, void *ctx_void)
114 {
115 ucontext_t *ctx = (ucontext_t*)ctx_void;
116
117 if (rip != ctx->uc_mcontext.gregs[REG_RIP]) {
118 printf("[FAIL]\tExpected SIGSEGV at 0x%lx but got RIP=0x%lx\n",
119 rip, (unsigned long)ctx->uc_mcontext.gregs[REG_RIP]);
120 fflush(stdout);
121 _exit(1);
122 }
123
124 siglongjmp(jmpbuf, 1);
125 }
126
test_syscall_fallthrough_to(unsigned long ip)127 static void test_syscall_fallthrough_to(unsigned long ip)
128 {
129 void *new_address = (void *)(ip - 4096);
130 void *ret;
131
132 printf("[RUN]\tTrying a SYSCALL that falls through to 0x%lx\n", ip);
133
134 ret = mremap((void *)current_test_page_addr, 4096, 4096,
135 MREMAP_MAYMOVE | MREMAP_FIXED, new_address);
136 if (ret == MAP_FAILED) {
137 if (ip <= (1UL << 47) - PAGE_SIZE) {
138 err(1, "mremap to %p", new_address);
139 } else {
140 printf("[OK]\tmremap to %p failed\n", new_address);
141 return;
142 }
143 }
144
145 if (ret != new_address)
146 errx(1, "mremap malfunctioned: asked for %p but got %p\n",
147 new_address, ret);
148
149 current_test_page_addr = new_address;
150 rip = ip;
151
152 if (sigsetjmp(jmpbuf, 1) == 0) {
153 asm volatile ("call *%[syscall_insn]" :: "a" (SYS_getpid),
154 [syscall_insn] "rm" (ip - 2));
155 errx(1, "[FAIL]\tSyscall trampoline returned");
156 }
157
158 printf("[OK]\tWe survived\n");
159 }
160
main()161 int main()
162 {
163 /*
164 * When the kernel returns from a slow-path syscall, it will
165 * detect whether SYSRET is appropriate. If it incorrectly
166 * thinks that SYSRET is appropriate when RIP is noncanonical,
167 * it'll crash on Intel CPUs.
168 */
169 sethandler(SIGUSR1, sigusr1, 0);
170 for (int i = 47; i < 64; i++)
171 test_sigreturn_to(1UL<<i);
172
173 clearhandler(SIGUSR1);
174
175 sethandler(SIGSEGV, sigsegv_for_fallthrough, 0);
176
177 /* One extra test to check that we didn't screw up the mremap logic. */
178 test_syscall_fallthrough_to((1UL << 47) - 2*PAGE_SIZE);
179
180 /* These are the interesting cases. */
181 for (int i = 47; i < 64; i++) {
182 test_syscall_fallthrough_to((1UL<<i) - PAGE_SIZE);
183 test_syscall_fallthrough_to(1UL<<i);
184 }
185
186 return 0;
187 }
188